Method for producing low friction fluorine rubber crosslinked body

ABSTRACT

A fluororubber composition includes a polyol-crosslinkable fluororubber, a crosslinking accelerator, a polyol crosslinking agent and calcium hydroxide, with the crosslinking accelerator having a specific weight ratio relative to the polyol crosslinking agent, and is heat treated under specific conditions to give a low-friction fluororubber crosslinked product that is well balanced and excellent in properties such as low frictional properties, low tackiness and low resilience properties and that is favorably employed as rubber vibration insulators and impact-absorbing stoppers represented by HDD stoppers.

This is a nationalization of PCT/JP2004/005683 filed 21 Apr. 2004 andpublished in Japanese.

FIELD OF THE INVENTION

The present invention relates to a process for producing a low-frictionfluororubber crosslinked product, more particularly to a process forproducing a low-friction fluororubber crosslinked product well balancedand excellent in properties such as low frictional properties, lowtackiness and low resilience properties and favorably employed forfabrication of parts including rubber vibration insulators andimpact-absorbing stoppers, particularly parts for HDD (hard disk drive)head controllers. The invention further relates to uses of alow-frication fluororubber crosslinked product obtained by the process.

BACKGROUND OF THE INVENTION

Fluororubbers possess rubber elasticity, an inherent characteristic incommon with other general-purpose rubbers, and are superior to thegeneral-purpose rubbers in properties such as resistance to heat, oiland chemicals. Such properties provide various applications, includingleakproof rubber parts represented by O-rings, packings and gaskets,rubber vibration insulators, belts and rubber-coated fabrics, as well asimpact-absorbing stoppers for printer head controllers and HDD (harddisk drive) head controllers, more particularly stoppers fitted in HDDto prevent a malfunction of a reading arm.

The conventional fluororubbers have surface tackiness and high frictioncoefficient, and therefore the processing thereof often involves tackeliminating treatment on the crosslinked rubber surface. This increasesthe treatment cost.

The use of conventional fluororubbers as stoppers of storage devices inhard disk drives (HDD) causes a malfunction by the tackiness of thestopper to the arm. Furthermore, their damping properties are greatlydependent on temperature and the impact resilience is increased at hightemperatures such that the stoppers do not absorb the vibration of thearm. As used herein, the stoppers are parts that define the range ofmovement (swing) to which an arm having a reading head at the tip canmove from a standby position and that absorb the impact to prevent amalfunction of the arm after operation or standby.

Meanwhile, recent rubber stoppers that are increasingly used includemagnet holder-type stoppers that incorporate a magnet in the rubber tofix the arm by magnetic attraction, and crush stop-type stoppersarranged on both sides of the arm. Performances required for suchstoppers are mainly the following three:

(1) The stoppers exhibit excellent impact-absorbing properties when thearm collides therewith.

(2) The rubber stopper holds an arm end portion (metal) in close contactby the magnetic force or the like during standby but does not adherethereto.

(3) The stoppers are clean.

The stoppers of common fluororubbers are generally satisfactory in termsof (1) the impact-absorbing properties and (3) the cleanness, but theyhave high tackiness and fail to satisfy the required performance (2).

To satisfy the required performance (2), JP-B-H04-37094 proposes amethod in which the rubber surface is impregnated with a solution of acrosslinking agent and a crosslinking accelerator for fluororubber toperform re-crosslinking and thereby the surface tackiness is eliminated.This method, however, uses a large amount of solvent and is undesirablein view of adverse effects on the environment. Moreover, the solventlimits the impregnation, so that products, for example stoppers, havevariations in performance and some cause a malfunction of HDD.

The present inventors studied diligently in order to solve the aforesaidproblems, and have arrived at an uncrosslinked fluororubber compositionthat comprises a polyol-crosslinkable fluororubber in combination with aspecific crosslinking agent, crosslinking accelerator, calcium hydroxideand optionally magnesium oxide, wherein the crosslinking accelerator hasa weight ratio (R) to the crosslinking agent (crosslinkingaccelerator/crosslinking agent) in the range of 0.9 to 5, and that whenheat treated under specific conditions (the heat treatment may bepreceded by polyol crosslinking and preforming according to need), thecrosslinking accelerator in the fluororubber composition (or a preform)favorably migrates to the superficial layer to increase the crosslinkingdensity of the rubber surface such that the rubber surface displaysreduced frictional properties and tackiness and the fluororubber formedproduct exhibits low impact resilience.

Specifically, the present inventors have found that the greater thecrosslinking accelerator (such as an organic quaternary phosphoniumsalt)/crosslinking agent ratio is within the range of 0.9 to 5, thelarger the amount of the crosslinking accelerator migrating to therubber surface, with the result that the crosslinking density increasesin the surface of rubber formed product while that of the entire rubberformed product decreases, leading to lowered impact resilience.

The present inventors have also found that the fluororubber compositionin which the crosslinking accelerator (such as an organic quaternaryphosphonium salt)/crosslinking agent ratio is higher than in thetraditional fluororubber compositions can give a low-frictionfluororubber crosslinked product that is well balanced and improved inproperties such as low frictional properties, low tackiness and lowresilience properties and is favorably employed for fabrication of partsincluding leakproof rubber parts represented by O-rings, packings andgaskets, rubber vibration insulators, belts, rubber-coated fabrics, andimpact-absorbing stoppers as stoppers in HDD. The present invention hasbeen completed based on these findings.

Known technologies related to the polyol vulcanization of fluororubbersinclude the following.

JP-B-H04-37094 discloses a process of surface modification of afluorine-containing elastomer formed product wherein a surface of avulcanized fluorine-containing elastomer formed product is impregnatedwith a crosslinking (vulcanizing) agent polyhydroxy compound andoptionally a vulcanization accelerating promoter (crosslinkingaccelerator) to perform re-vulcanization. This surface modificationprocess is described to provide non-tackiness and low frictionalproperties in the surface of vulcanized fluorine-containing elastomerformed product.

However, the process employs a surface treatment solution of thecrosslinking agent and crosslinking accelerator in an organic solventsuch as acetone for impregnating the surface with the crosslinking agentpolyhydroxy compound and vulcanization accelerating promoter(crosslinking accelerator). The use of organic solvent possibly leads toenvironmental pollution. Moreover, the surface treatment aftervulcanization adds a process to increase the cost.

JP-A-H07-3099 discloses a fluororubber composition comprising (A) 100parts by weight of a fluororubber obtained by copolymerizing vinylidenefluoride, hexafluoropropylene and optionally tetrafluoroethylene, (B)0.05 to 2 parts by weight of an organic quaternary phosphonium salt, (C)0.01 to 3 parts by weight of a nitrogen-containing organic compoundand/or a phosphorous-containing organic compound, (D) 0.1 to 10 parts byweight of a polyhydroxy compound, and (E) 0.5 to 30 parts by weight of ametal oxide and/or a metal hydroxide. It is described that thecomposition may be preformed into a desired shape, be placed in a mold,and be compression molded under heating to perform polyol vulcanization,giving a vulcanized fluororubber formed product. The thus-formed productis described to maintain mechanical characteristics and be free offorming failure during the vulcanization forming.

However, the formed products disclosed in Examples of the patentdocument show insufficient migration of the organic quaternaryphosphonium salt to the surface of rubber formed product, andconsequently have high friction coefficient and tackiness.

JP-A-H07-3100 discloses a fluororubber composition comprising (A) 100parts by weight of a fluororubber obtained by copolymerizing vinylidenefluoride, hexafluoropropylene and optionally tetrafluoroethylene, (B)0.05 to 2 parts by weight of an organic quaternary phosphonium salt, (C)0.01 to 3 parts by weight of an organic quaternary ammonium hydrogensulfate, (D) 0.1 to 10 parts by weight of a polyhydroxy compound, and(E) 0.5 to 30 parts by weight of a metal oxide and/or a metal hydroxide.It is described that the composition may be preformed into a desiredshape, be placed in a mold, and be compression molded under heating toperform polyol vulcanization, giving a vulcanized fluororubber formedproduct. The thus-formed product is described to have good mechanicalproperties and be free of forming failure during the vulcanizationforming.

However, as in the case of the above patent document 2, the formedproducts disclosed in Examples of this patent document show insufficientmigration of the organic quaternary phosphonium salt to the surface ofrubber formed product, and consequently have high friction coefficientand tackiness.

JP-A-H07-82449 discloses a polyol-vulcanizable fluororubber compositioncomprising a polyol-vulcanizable fluororubber and a compound analogousto hydrotalcite. According to the patent document, vulcanization isperformed in a manner such that the fluororubber is mixed withvulcanizing ingredients and the mixture is subjected to primaryvulcanization by press vulcanization at 140 to 200° C. for about 2 to120 minutes and to secondary vulcanization by oven vulcanization atabout 150 to 250° C. for about 0 to 30 hours. The vulcanizingingredients mentioned in the patent document include the following,which are used in the amounts described per 100 parts by weight of thefluororubber: a vulcanizing agent (e.g., a polyhydroxy aromaticcompound) used in an amount of 0.5 to 10 parts by weight, an acidreceiver (e.g., a bivalent metal oxide or hydroxide) used in an amountof 1 to 20 parts by weight, and a vulcanization accelerator (e.g., aquaternary ammonium or phosphonium salt) used in an amount not more than10 parts by weight. The composition is described to have good moldreleasability, excellent vulcanization properties and improved engineoil resistance.

However, the formed products disclosed in Examples of this patentdocument show insufficient migration of the vulcanization accelerator(such as a quaternary phosphonium salt) to the surface of rubber formedproduct, and consequently have high friction coefficient and tackiness.Moreover, the use of the vulcanization accelerator, such as an organicquaternary phosphonium salt, in an increased amount in combination withthe hydrotalcite-analogous compound as described in the patent documentresults in a higher crosslinking rate such that the composition iscrosslinked before being poured into a mold in the press molding andcannot be formed into a desired shape.

JP-A-2000-34379 discloses a fluororubber composition comprising araw-material fluororubber, a polyol vulcanizing agent, an organicpromoter, a vulcanization supplement accelerator, an acid receiver andoptionally a filler, wherein the vulcanization supplement accelerator iscalcium hydroxide that is treated with a fatty acid ester or the likeand has an average particle diameter of not more than 7.5 μm and aspecific surface area of not less than 20 m²/g. The composition isdescribed to give a vulcanized product excellent in resistance topermanent compression set. According to the patent document, thevulcanization is performed in a manner such that primary vulcanizationis carried out at 170° C. for 10 minutes and secondary vulcanization at200° C. for 24 hours.

However, fluororubber formed products obtained according to this patentdocument have increased crosslinking density and impact resilience,presumably because of the fact that the vulcanization supplementaccelerator has a high specific surface area and is treated with a fattyacid ester or the like. Moreover, the formed products show insufficientmigration of the organic accelerator (vulcanization accelerator) such asan organic quaternary phosphonium salt to the surface of rubber formedproduct. Furthermore, Examples in this patent document result in highfriction coefficient and tackiness.

JP-A-2001-192482 discloses a process in which a fluororubber compositionis vulcanized and formed in the presence of a polyol vulcanizing agentand is heat treated at a temperature of about 250 to 300° C. to give avulcanized fluororubber formed product having superior properties suchas resistance to permanent compression set, wherein the fluororubbercomposition comprises 100 parts by weight of a fluororubber, 0.5 to 3parts by weight of calcium hydroxide, 4 to 15 parts by weight ofmagnesium oxide, and 10 to 50 parts by weight of thermal black and abituminous coal filler combined. The process employs the polyolvulcanizing agent, such as a polyhydroxy aromatic compound, in an amountof about 0.5 to 10 parts by weight per 100 parts by weight of thefluorine-containing rubber, and an ammonium or phosphonium salt in anamount of about 0.1 to 30 parts by weight. According to the patentdocument, the fluororubber composition is vulcanized and formed using acompression press or the like at about 150 to 230° C. for about 1 to 30minutes; the products for use as grommets and seal packings are furtherheat treated (secondary vulcanization) at about 250 to 300° C. for about5 to 48 hours in an air oven or the like.

However, the formed products disclosed in this patent document have highimpact resilience, and show insufficient migration of the organicquaternary phosphonium salt to the rubber surface and consequentlyexhibit high friction coefficient and tackiness.

The fluororubber compositions and vulcanized formed products thereofproposed so far further include the following.

Japanese Patent No. 3063172 (corresponding to JP-A-H04-236254) disclosesa fluororubber composition comprising 100 parts by weight of afluororubber and 0.5 to 10 parts by weight of a liquid hydrocarbonrubber selected from liquid polyisoprene rubbers and hydrogenated liquidpolyisoprene rubbers. The composition is described to be excellent inprocessability such as extrusion properties and in vulcanized rubberproperties.

Japanese Patent No. 3222054 (corresponding to JP-A-H09-208751) disclosesa rubber composition comprising (A) a fluororubber polymer, (B) a waxcontaining a fluorine-containing organic group and having a meltingpoint of 30 to 200° C., and (C) a crosslinking agent selectedfromamines, polyols and peroxides. The rubber composition is describedto possess excellent workability, kneading processability and moldreleasability, and to give a formed product comparable in properties tothe existing products.

Japanese Patent No. 2653340 (corresponding to JP-A-H06-293850) disclosesa fluororubber composition comprising (A) a polyol-crosslinkablefluororubber, (B) a liquid fluororubber, and (C) a polyol in which atleast one OH group in the molecule is silylated. It also describes thatthe composition may be crosslinked at a low temperature of 130 to 160°C. (primary crosslinking) to avoid foaming, and be subjected tosecondary crosslinking at a temperature of 120 to 250° C. Thecomposition is described to be excellent in workability and to give aformed product having low hardness and widespread uses.

JP-A-H05-239300 discloses a vulcanizable fluoroelastomer compositioncomprising (A) an elastomer copolymer having a vinylidene fluoride unitand at least one fluorine-containing monomer unit, (B) a tertiaryphosphine vulcanization accelerator, such as triphenylphosphine,substituted with an alkoxyl or phenoxy group, (C) a polyol crosslinkingagent, and (D) a bivalent metal oxide or hydroxide. The composition issubjected to primary vulcanization (press vulcanization) and secondaryvulcanization (oven heating) to provide a vulcanized formed product,such as a sealing material. The primary vulcanization rate is high, andthe vulcanized formed product has good rubber elasticity and tensileproperties.

JP-A-H06-248145 discloses a fluororubber composition comprising afluororubber, calcium oxide and polyethylene wax, wherein thefluororubber is obtained by copolymerizing vinylidene fluoride,tetrafluoroethylene and propylene. The patent document teaches that thecomposition may be vulcanized at 100 to 400° C. for several seconds to 5hours, and be subjected to secondary vulcanization at 150 to 300° C. forabout 30 minutes to 48 hours to stabilize properties of the vulcanizate.According to the disclosure of the document, the composition is free offusion failure in the vulcanization and forming, and the formed producthas high heat resistance.

JP-A-H06-306180 discloses a process for producing a vulcanizedfluororubber formed product, wherein a fluororubber obtained bycopolymerizing vinylidene fluoride, hexafluoropropylene and optionallytetrafluoroethylene, is formed by polyol vulcanization in a mold withuse of an organic quaternary ammonium salt as vulcanization accelerator.The document describes that the vulcanization forming can be performedwithout forming failure.

The fluororubber composition disclosed in these patent documents includea liquid hydrocarbon rubber, a wax containing a fluorine-containingorganic group and having a melting point of 30 to 200° C., a liquidfluororubber, a tertiary phosphine crosslinking accelerator, calciumoxide, a polyethylene wax, or an organic quaternary ammonium salt.Vulcanized formed products from the fluororubber compositions areunsatisfactory in low tackiness, low frictional properties and lowimpact resilience, and in balance of such properties.

DISCLOSURE OF THE INVENTION

A process for producing a low-friction fluororubber crosslinked productaccording to the present invention comprises:

preliminarily polyol crosslinking a polyol-crosslinkable fluororubbercomposition according to need, the composition comprising apolyol-crosslinkable fluororubber in combination with a crosslinkingaccelerator (preferably an organic quaternary phosphonium salt), apolyol crosslinking agent (preferably a bisphenol), calcium hydroxideand optionally magnesium oxide, wherein the crosslinking accelerator hasa weight ratio (R) to the polyol crosslinking agent (crosslinkingaccelerator/polyol crosslinking agent) in the range of 0.9 to 5,preferably 0.9 to 3, more preferably 0.9 to 2; and

heat treating the fluororubber composition at a temperature in the rangeof 150 to 300° C., preferably 200 to 300° C., more preferably 240 to300° C., for 0.1 to 48 hours, preferably 1 to 48 hours, more preferably10 to 48 hours to produce a low-friction fluororubber crosslinkedproduct having a surface frication coefficient of less than 1.

The polyol-crosslinkable fluororubber composition used in the inventionpreferably contains the crosslinking accelerator and the polyolcrosslinking agent in amounts of 2.1 to 20 parts by weight and 0.4 to 20parts by weight, respectively, and calcium hydroxide having a specificsurface area of less than 20 m²/g in an amount of 0.5 to 10 parts byweight, per 100 parts by weight of the polyol-crosslinkablefluororubber.

The polyol-crosslinkable fluororubber composition used in the inventionpreferably contains the magnesium oxide in an amount of not more than3.0 parts by weight, per 100 parts by weight of the polyol-crosslinkablefluororubber.

The polyol-crosslinkable fluororubber composition used in the inventionpreferably contains polytetrafluoroethylene (PTFE) in an amount of 5 to200 parts by weight, per 100 parts by weight of the polyol-crosslinkablefluororubber.

In the present invention, the polyol-crosslinkable fluororubbercomposition is polyol-crosslinked using a compression mold whose innerperipheral surface is unleveled to give a crosslinked product having anuneven surface with an average depth of 0.5 to 200 μm, and thecrosslinked product is subjected to the heat treatment.

When a crosslinked product from the polyol-crosslinkable fluororubbercomposition is to be used as a stopper in hard disk drive (HDD), thepolyol crosslinking agent is preferably contained in an amount of 1 to10 parts by weight per 100 parts by weight of the polyol-crosslinkablefluororubber, and the weight ratio R (crosslinking accelerator/polyolcrosslinking agent) is preferably in the range of 0.9 to 2.

An impact-absorbing stopper represented by a HDD stopper according tothe present invention is obtained by the process for producing alow-friction fluororubber crosslinked product as described above.

The crosslinked product used as HDD stopper preferably has a change ofholding torque of not more than 14%.

The present invention provides an inexpensive process whereby alow-frication fluororubber crosslinked product can be produced simply byheat treating a fluororubber composition (the heat treatment may bepreceded by vulcanization and preforming according to need) withoutsurface modification with a solution containing an organic solventthereby to reduce the environmental pollution, and wherein thelow-frication crosslinked fluororubber obtained is well balanced andexcellent in properties such as low frictional properties, low tackinessand low resilience properties and is favorably employed for fabricationof parts including rubber vibration insulators, impact-absorbingstoppers, fluid leak-proof rubber parts, belts, rubber-coated fabricsand wipers. The low-friction fluororubber crosslinked product accordingto the invention has improved and stable non-tackiness properties andcan be suitably used as stoppers in HDD.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of a testing device used for evaluating alow-friction fluororubber crosslinked product obtained by the presentprocess for non-tackiness durability;

FIG. 2 is a sectional view of an outer stopper fitted in the testingdevice of FIG. 1;

FIG. 3 is an explanatory view of a testing device used for evaluatingthe change of tackiness in a magnet test; and

FIG. 4 is an explanatory view of a testing device used for evaluatingthe holding torque.

PREFERRED EMBODIMENTS OF THE INVENTION

The process for producing a low-friction fluororubber crosslinkedproduct according to the present invention will be described in detailhereinbelow.

Process for Producing Low-friction Fluororubber Crosslinked Product

The process for producing a low-friction fluororubber crosslinkedproduct according to the present invention comprises:

preliminarily polyol crosslinking a polyol-crosslinkable fluororubbercomposition (discussed later) according to need; and

heat treating the crosslinked product (crosslinked fluororubber) at atemperature in the range of 150 to 300° C., preferably 200 to 300° C.,more preferably 240 to 300° C., for 0.1 to 48 hours, preferably 1 to 48hours, more preferably 10 to 48 hours.

Hereinbelow, the polyol-crosslinkable fluororubber composition(fluororubber composition crosslinkable with a polyol) favorably used inthe invention, vulcanization conditions (of primary vulcanization andsecondary vulcanization performed according to need), conditions ofpost-vulcanization heat treatment, and properties of the obtainablelow-friction fluororubber crosslinked product will be sequentiallydiscussed.

(Polyol-crosslinkable Fluororubber Composition)

The polyol-crosslinkable fluororubber composition (fluororubbercomposition crosslinkable with a polyol) suitable for use in the presentinvention contains a polyol-crosslinkable fluororubber, a crosslinkingaccelerator organic quaternary phosphonium salt, a polyol crosslinkingagent represented by a bisphenol, calcium hydroxide, and optionallymagnesium oxide.

Polyol-crosslinkable Fluororubber

The invention may employ one or more kinds of fluorine-containing olefin(co)polymers as the polyol-crosslinkable fluororubbers (fluororubberscrosslinkable with a polyol).

The fluorine-containing olefins include vinylidene fluoride,hexafluoropropylene, pentafluoropropylene, trifluoroethylene,trifluorochloroethylene, tetrafluoroethylene, vinyl fluoride,perfluoroacrylic esters, perfluoroalkyl acrylates, perfluoromethyl vinylether and perfluoropropyl vinyl ether. These fluorine-containing olefinsmay be used singly or in combination of two or more kinds.

Preferred polyol-crosslinkable fluororubbers ((co)polymers) includecommercially available fluororubbers such as vinylidenefluoride/hexafluoropropylene binary copolymer,tetrafluoroethylene/propylene binary copolymer, and vinylidenefluoride/tetrafluoroethylene/hexafluoropropylene ternary copolymer.

Crosslinking Agent

The invention preferably employs polyol crosslinking agent bisphenols.Specific examples include polyhydroxy aromatic compounds such as2,2-bis(4-hydroxyphenyl)propane[bisphenol A],2,2-bis(4-hydroxyphenyl)perfluoropropane[bisphenol AF],bis(4-hydroxyphenyl)sulfone[bisphenol S], bisphenol A-bis(diphenylphosphate), 4,4′-dihydroxydiphenyl, 4,4′-dihydroxydiphenyl methane and2,2-bis(4-hydroxyphenyl)butane. Preferred are bisphenol A and bisphenolAF. They may be in the form of alkali metal salts or alkaline earthmetal salts. Also employable as the crosslinking agents are commerciallyavailable master batches containing a raw-material rubber and acrosslinking agent, with examples including CURATIVE VC 30 (manufacturedby DuPont Dow Elastomers, containing 50 wt % of crosslinking agentbisphenol AF). These crosslinking agents may be used singly or incombination of two or more kinds.

Crosslinking Accelerator

The crosslinking accelerators include those commonly used in the polyolcrosslinking, such as organic quaternary phosphonium salts, activehydrogen-containing aromatic compound/quaternary phosphonium saltequimolar molecular compounds, and bivalent metal/amine complexcompounds. These crosslinking accelerators may be used singly or incombination of two or more kinds. Of these crosslinking accelerators,the organic quaternary phosphonium salts are preferable in view ofreduction of outgassing from the low-friction fluororubber crosslinkedproduct.

Specific examples of the crosslinking accelerator organic quaternaryphosphonium salts are described in paragraphs [0010] to [0012] ofJP-A-2001-192482 to the present applicant, withtriphenylbenzylphosphonium bromide and triphenylbenzylphosphoniumchloride being preferred. Also employable as the crosslinkingaccelerators are commercially available master batches containing araw-material rubber and a crosslinking accelerator, with examplesincluding CURATIVE VC 20 (manufactured by DuPont Dow Elastomers,containing 33 wt % of crosslinking accelerator organic phosphoniumsalt).

Other Ingredients

Where necessary, the polyol-crosslinkable fluororubber composition mayappropriately contain further ingredients common in the rubber industryin addition to the aforementioned, while still achieving the effects ofthe crosslinking agent and crosslinking accelerator used in theinvention. Such ingredients include reinforcing agents such as carbonblacks and carbon fibers; fillers such as calcium carbonate,magnesiumcarbonate, aluminumhydroxide, magnesiumhydroxide, aluminumsilicate, magnesium silicate, calcium silicate, potassium titanate,titanium oxide, barium sulfate, aluminum borate, glass fibers and aramidfibers; processing aids such as waxes and metallic soaps; acid receiverssuch as zinc oxide (excluding calcium hydroxide, the same applieshereinafter); anti-aging agents; and thermoplastic resins. Silica canprovide an effect of lowering tackiness at room temperature, butincreases the tackiness to metals at a high humidity, so that the nonuseof silica is sometimes preferable.

The polyol-crosslinkable fluororubber composition (fluororubbercomposition crosslinkable with a polyol) desirably contains theingredients in the following amounts per 100 parts by weight of thepolyol-crosslinkable fluororubber:

2.1 to 20 parts by weight, preferably 2.5 to 10 parts by weight of thecrosslinking accelerator (preferably organic quaternary phosphoniumsalt);

0.4 to 20 parts by weight, preferably 1 to 10 parts by weight of thepolyol crosslinking agent (preferably bisphenol);

not more than 3 parts by weight, preferably 1 to 3 parts by weight ofmagnesium oxide according to need; and

0.5 to 10 parts by weight, preferably 1 to 7 parts by weight,particularly preferably 1 to 5 parts by weight of calcium hydroxide.

Any amount of crosslinking accelerator less than the aforementionedtends to increase the friction coefficient and tackiness of the rubbersurface. Any amount exceeding the aforementioned tends to result in alow-friction fluororubber crosslinked product (formed product) liable tocracks upon flexing, compressing or distorting.

Any amount of crosslinking agent less than the. aforementioned tends tocause foams after forming and the crosslinked products cannot achieve auniform shape. Any amount exceeding the aforementioned tends to resultin a low-friction fluororubber crosslinked product (formed product)liable to cracks upon flexing, compressing or distorting.

In the event that foaming occurs during the crosslinking and formingeven when the crosslinking agent is used in the amount specified above,the amounts of the reinforcing agent, filler, acid receiver andcrosslinking agent are increased and appropriately adjusted according toneed to solve the foaming problem.

In the polyol-crosslinkable fluororubber composition, the crosslinkingaccelerator represented by the organic quaternary ammonium salts andorganic quaternary phosphonium salts has a weight ratio (R) to thepolyol crosslinking agent represented by the bisphenols (crosslinkingaccelerator/polyol crosslinking agent) in the range of 0.9 to 5,preferably 0.9 to 4, more preferably about 0.9 to 3, particularlypreferably 0.9 to 2.

In the polyol-crosslinkable fluororubber composition, any weight ratio R(crosslinking accelerator/polyol crosslinking agent) less than theaforementioned tends to lead to insufficient migration of thecrosslinking accelerator to the rubber surface, and consequently therubber surface cannot have an adequate crosslinking density and fails toachieve desired low tackiness and low frictional properties. The weightratio R exceeding the aforementioned can provide an increasedcrosslinking density of the rubber surface but tends to result in alow-friction fluororubber crosslinked product (formed product) liable tocracks upon flexing, compressing or distorting.

Excessive use of magnesium oxide (more than 3.0 parts by weight per 100parts by weight of the uncrosslinked fluororubber) results in a formedproduct having high impact resilience. Further, because magnesium oxideper se exhibits tackiness to metals, the excessive use thereof causesthe rubber formed product obtained to display higher tackiness to amating metal. Furthermore, when magnesium oxide is used excessively, thecrosslinking accelerator such as organic phosphonium salt makesinsufficient migration to the surface of formed product, andconsequently desired low friction coefficient and low tackiness cannotbe achieved.

When the low-friction fluororubber crosslinked product is to be used asa stopper in hard disk drive, the crosslinking agent is contained in anamount of 0.4 to 20 parts by weight, preferably 0.4 to 10 parts byweight per 100 parts by weight of the polyol-crosslinkable fluororubber,and the weight ratio R (crosslinking accelerator/polyol crosslinkingagent) is in the range of 0.9 to 2, preferably 0.9 to 1.5. When theamount of crosslinking agent and the weight ratio R are in the aboveranges, the fluororubber crosslinked product obtained has excellentsurface non-tackiness and stability of non-tackiness, and can besuitably used as a HDD stopper.

The lower limit of the amount of magnesium oxide may be 0 part by weightper 100 parts by weight of the uncrosslinked fluororubber. Preferably,the amount is in the range of 1 to 3.0 parts by weight, in which casethe crosslinking can take place at an adequate crosslinking rate and canproduce a foam-free crosslinked product (formed product) having lowfrictional properties and low tackiness. The amount of magnesium oxideshows a tendency similar to that of calcium hydroxide.

The use of calcium hydroxide in the above-mentioned amount provides anadequate crosslinking density and low impact resilience, and can reducethe possibility of foaming during the forming.

In general, the use of calcium hydroxide in smaller amounts tends tolead to lower crosslinking density and impact resilience, and increasesthe possibility of foaming during the forming.

In the event that foaming occurs during forming, an acid receiver isdesirably added in an increased amount, for example in an amount of 0.5to 10 parts by weight per 100 parts by weight of thepolyol-crosslinkable fluororubber.

The use of calcium hydroxide exceeding the aforementioned rangeincreases the crosslinking density of entire rubber product and theimpact resilience, and the crosslinking accelerator makes remarkablyreduced and deteriorated migration to the rubber surface and often failsto achieve low surface tackiness.

The invention can employ commercially available calcium hydroxides,preferably having a specific surface area of less than 20 m²/g, as theyare.

The use of calcium hydroxide with a specific surface area exceeding 20m²/g increases the crosslinking density of entire rubber product and theimpact resilience, and the crosslinking accelerator makes remarkablyreduced and deteriorated migration to the rubber surface and often failsto achieve low surface tackiness.

In view of low frictional properties, low tackiness and low impactresilience, the polyol-crosslinkable fluororubber composition used inthe invention may contain polytetrafluoroethylene (PTFE) in an amount of5 to 200 parts by weight, preferably about 20 to 100 parts by weight,per 100 parts by weight of the polyol-crosslinkable fluororubber.

The polytetrafluoroethylene (PTFE) per se has superior low frictionalproperties, low tackiness and good impact resilience properties, and theaddition thereof in the preparation of the fluororubber composition canimprove low frictional properties, low tackiness and low impactresilience of formed products obtainable.

Any amount of PTFE less than the aforementioned tends to lead to a pooreffect of lowering the impact resilience of formed products obtainable,and that exceeding the aforementioned tends to lead to excessively highhardness of formed products obtainable such that the rubber elasticityis lost.

Preparation and Vulcanization of Polyol-crosslinkable FluororubberComposition

To prepare the polyol-crosslinkable fluororubber composition, a compoundcontaining the aforementioned ingredients in the specified amounts maybe kneaded using an internal kneading machine such as an internal mixer,a kneader or a Banbury mixer, or a common rubber-kneading machine suchas an open roll mill. An alternative method of preparation of thecomposition is to dissolve the ingredients separately in solvents and todisperse them using a stirring machine or the like.

The polyol-crosslinkable fluororubber composition obtained as describedabove may be crosslinked (vulcanized) and formed with use of aninjection molding machine, a compression molding machine, a vulcanizingpress or an oven, at a temperature of 140 to 230° C. for about 1 to 120minutes (primary vulcanization). The primary vulcanization is a step inwhich the composition is crosslinked to an extent such that a specificshape can be maintained (preforming). For a complicated shape, thepreforming preferably involves a mold. The primary vulcanization may beperformed with an air oven or the like.

In the present invention, the polyol-crosslinkable fluororubbercomposition may be polyol-crosslinked, particularly in the primaryvulcanization, using a compression mold as the vulcanizing press or incombination with the vulcanizing press, wherein the inner peripheralsurface of the mold is unleveled to a certain depth to give acrosslinked product having an uneven surface with a desired depth (e.g.,0.5 to 200 μm). The crosslinked product having an uneven surface with anaverage depth of 0.5 to 200 μm can exhibit improved low frictionalproperties and low tackiness.

After kneaded, the polyol-crosslinkable fluororubber composition iscompression molded as described later. The kneaded composition may: (a)be cooled to room temperature, then reheated and compression molded; or(b) be heated continuously from the kneading and compression molded. Thecompression processing with a compression molding machine generallyadopts the step (a) for a processing reason.

For the production of fluororubber formed products such as rubber hoses,the kneaded fluororubber composition is extruded into a tube and issubjected to oven vulcanization, in which case the step (b) is adopted.

Shaping the polyol-crosslinkable fluororubber composition into a desiredshape before vulcanization permits production of a low-friction andlow-tackiness formed product by whichever (a) or (b). The levels of lowfrictional properties and low tackiness of the fluororubber crosslinkedproduct is affected not by the temperature rise pattern or curve forheat treatment but by the temperature and time of the heat treatment.

The heat treatment method in the invention may be similar to that forordinary secondary vulcanization. But ordinary secondary vulcanizationcannot achieve low frictional properties and low tackiness unless thecomposition is as described in the invention (polyol-crosslinkablefluororesin composition). With the conventional fluororubbers, thesecondary vulcanization has purposes of completing the crosslinkingreaction that has taken place inadequately in the primary vulcanization,and gasifying low-molecular components in the rubber to improve thestrength and reduce the permanent compression set. On the other hand,the secondary vulcanization in the present invention is intended tocause the crosslinking accelerator in the primary vulcanizate to migrateto the surface and thereby to achieve low frictional properties and lowtackiness and to cure the surface.

(Post-vulcanization Heat Treatment)

In the invention, the polyol-crosslinkable fluororubber composition isheat treated at a temperature in the range of 150 to 300° C., preferably200 to 300° C., more preferably 240 to 300° C., for 0.1 to 48 hours,preferably 1 to 48 hours, more preferably 10 to 48 hours.

In view of prevention of outgassing of low-molecular volatile componentsfrom the crosslinked product, the polyol-crosslinkable fluororubbercomposition may be subjected to the preliminary polyol crosslinking(vulcanization) as described above according to need, and the resultantcrosslinked product may be heat treated at a temperature in the range of150 to 300° C., preferably 200 to 300° C., more preferably 240 to 300°C., for 0.1 to 48 hours, preferably 1 to 48 hours, more preferably 10 to48 hours.

When the crosslinked product contains much low-molecular volatile(outgassing) components, the use thereof as a HDD stopper or the likecan cause contamination of metal parts such as disks by thelow-molecular components volatilizing from the crosslinked product.Accordingly, the heat treatment is preferably performed at hightemperatures and over a long period of time.

This heat treatment of the crosslinked product causes the crosslinkingaccelerator or the like in the product to migrate slowly from the insideto the superficial surface, so that the superficial surface (forexample, the area from the surface to a depth of about 100 μm of thecrosslinked product) has a crosslinking density higher than that of theinside (for example, core) of the crosslinked product. As a result, theheat-treated crosslinked product can display a surface (crosslinkedproduct surface) having reduced tackiness, low frictional properties andlow impact resilience. The heat treatment performed under theaforementioned conditions can provide not only superior non-tackinessbut also excellent stability of non-tackiness of the surface offluororubber crosslinked product.

Specifically, the heat treatment of the crosslinked product produces alow-friction fluororubber crosslinked product having a surface staticfrication coefficient and dynamic friction coefficient of less than 1each, preferably from 0.1 to 0.7 each.

When the polyol-crosslinkable fluororubber composition (uncrosslinkedproduct) is heat treated under the conditions mentioned above, i.e., ata temperature in the range of 150 to 300° C., preferably 200 to 300° C.,more preferably 240 to 300° C., for 0.1 to 48 hours, preferably 1 to 48hours, more preferably 10 to 48 hours, the crosslinking reaction andmigration of the crosslinking accelerator to the superficial surfaceoccur simultaneously. The resultant low-friction fluororubbercrosslinked product is reduced in tackiness, frictional properties andimpact resilience.

The heat treatment may employ a heating apparatus such as an oven, avulcanizing furnace or a high-frequency heater in any of theaforementioned embodiments regardless of whether the preliminary polyolcrosslinking is performed or not.

The post-vulcanization heat treatment of the fluororubber compositionmay be similar to that for secondary oven vulcanization common in therubber industry. The fluororubber composition used in the invention andconventional fluororubber compositions are completely different iningredients composition and thus have different purposes and functionsof the post-vulcanization heat treatment. With the conventionalfluororubbers (vulcanized formed products), the post-vulcanization heattreatment is mainly intended to improve tensile properties and reducepermanent compression set.

On the other hand, the invention employs the uncrosslinked fluororubbercomposition (or preform thereof) having the specific composition, andthe post-crosslinking heat treatment has purposes of quicklyfacilitating the migration of the crosslinking accelerator to the formedproduct surface, and stabilizing the accelerator to achievelow-frictional properties and low tackiness in the surface of formedproduct. The heat treatment does provide such effects.

With the conventional fluororubber compositions, primary vulcanizationsuch as press vulcanization in a mold under traditional conditions (asprimary vulcanization conditions of 140 to 200° C. and about 2 to 120minutes as disclosed in JP-A-H07-82449 (Patent Document 4)) cannotachieve adequate migration of the crosslinking accelerator to thesurface. Accordingly, the formed products obtainable do not show lowfrictional properties and low tackiness.

On the other hand, the uncrosslinked fluororubber composition of theinvention shows migration of the crosslinking accelerator to the surfacesimply upon primary vulcanization in an oven at a temperature in therange of 150 to 300° C., preferably 200 to 300° C., more preferably 240to 300° C., for 0.1 to 48 hours, preferably 1 to 48 hours, morepreferably 10 to 48 hours. The resultant low-friction fluororubbercrosslinked product has low frictional properties and low tackiness, andno further heat treatment under similar conditions is required.

Even when the conventional (uncrosslinked) fluororubbers are subjectedto primary vulcanization under similar conditions, they cannot givelow-friction and low-tackiness fluororubber crosslinked products.

In particular, the present invention can be distinguished fromJP-B-H04-37094 (Patent Document 1) as follows. As described inBACKGROUND OF THE INVENTION above, JP-B-H04-37094 discloses a method inwhich the surface of fluorine-containing elastomer vulcanizate isimpregnated with a solution (hereinafter the “treating agent”)containing a crosslinking (vulcanizing) agent for fluororubber andoptionally a vulcanization accelerating promoter (crosslinkingaccelerator) to perform re-vulcanization and thereby the crosslinkingdensity in the superficial surface of the fluorine-containing elastomerformed product is increased and the formed product is reduced intackiness and frictional properties.

In contrast, the present invention employs the specificpolyol-crosslinkable fluororubber composition which JP-B-H04-37094 andother patent documents never even suggest. The use of the compositionenables production of low-friction fluororubber formed products having asurface condition (e.g., low frictional properties) similar to thatdescribed in the patent document, simply by heat treatment of thecrosslinked product without any “treating agent” of JP-B-H04-37094.

Further, in the invention, the use of the crosslinking accelerator in anincreased amount leads to a low-friction fluororubber crosslinkedproduct capable of giving a heat-treated product with lower impactresilience at high temperatures.

Specifically, the crosslinked accelerator contained in the crosslinkedproduct makes migration to the rubber superficial surface upon heattreatment of the vulcanizate. The closer to the formed product surfacethe crosslinking accelerator migrates, the higher the crosslinkingdensity in the rubber superficial surface is. Accordingly, the inventioncan produce a low-friction fluororubber crosslinked product having asurface condition similar to that described in JP-B-H04-37094 simply byheat treating the crosslinked rubber formed product without coating therubber surface.

Furthermore, the invention can be distinguished from JP-A-2001-192482(Patent Document 6) as follows. JP-A-2001-192482 discloses a process inwhich a fluororubber composition is vulcanized and formed in thepresence of a polyol vulcanizing agent and is heat treated at atemperature of about 250 to 300° C. to give a vulcanized fluororubberformed product having superior properties such as resistance topermanent compression set, wherein the fluororubber compositioncomprises 100 parts by weight of a fluororubber, 0.5 to 3 parts byweight of calcium hydroxide, 4 to 15 parts by weight of magnesium oxide,and 10 to 50 parts by weight of thermal black and a bituminous coalfiller combined. The fluororubber composition disclosed in the patentdocument has a small crosslinking accelerator/crosslinking agent ratio,and contains much magnesium oxide, i.e., 4 to 15 parts by weight per 100parts by weight of the fluororubber. Even if a vulcanizate (crosslinkedrubber formed product) of this composition is heat treated under theconditions of the present invention, the organic quaternary phosphoniumsalt makes insufficient migration to the rubber surface and theresultant product has high impact resilience and high tackiness tometals.

(Low-friction Fluororubber Crosslinked Product)

The invention performs “heat treatment” of the fluororubber crosslinkedproduct from the crosslinkable fluororubber composition of specificingredients composition, thereby to produce a low-friction fluororubbercrosslinked product (fluororubber crosslinked heat-treated product)having a surface frication coefficient of less than 1, preferably from0.1 to 0.7. This fluororubber crosslinked product possesses a lowsurface friction coefficient as above, and further has low tackiness,low impact resilience, low static friction coefficient, low dynamicfriction coefficient, and appropriate hardness.

For example, the properties of the fluororubber crosslinked heat-treatedproduct are as follows:

The static friction coefficient (JIS P8147) is less than 1, preferablyin the range of 0.1 to 0.7, which indicates low tackiness.

The dynamic friction coefficient is less than 1, preferably in the rangeof 0.1 to 0.7, which indicates low tackiness. The dynamic frictioncoefficient is determined as follows: A 2 mm thick rubber sheet istested in accordance with JIS K 7125 and P 8147 with use of a surfaceproperty tester (manufactured by HEIDON). The testing conditions aresuch that a mating part is a 10 mm diameter chrome-plated steel ballfriction block, the moving rate is 50 mm/min, and the load is 50 g.

The hardness (determined with a Type A durometer in accordance with JISK 6253) is in the range of 40 to 85 (unit: POINT), preferably 60 to 80(unit: POINT).

The impact resilience is not more than 50%, preferably not more than 40%when determined as follows: A laminate consisting of 6 rubber sheetseach 2 mm in thickness and 29 mm in diameter is tested in accordancewith JIS K 6255 by the Lubke process at temperatures of 0° C., 25° C.and 70° C. This impact resilience permits the product to favorablyabsorb vibration of a mating part such as an arm.

The tackiness is not more than 100 g, preferably not more than 50 g whendetermined as follows: A metal rod (stainless steel made, weight: 16 g,configuration of curved surface contacting with the rubber: cylindricalcolumn 3 mm in diameter and 1.5 mm in width) is pressed against a 2 mmthick rubber sheet at 60° C. over a period of 24 hours or 72 hours, andthen at 0° C. for 24 hours. The tackiness of the rubber to the metal rodis measured at 0° C.

The non-tackiness durability (measurement conditions: described later)is not more than 100 g, preferably not more than 70 g. When the heattreatment is unperformed or inadequate, less crosslinking acceleratormakes migration to the formed product surface and the tackiness willincrease to 400 g or more.

Unleveling the surface of crosslinked product reduces the contact areabetween the low-friction fluororubber crosslinked product and the matingpart, and a friction coefficient of less than 1 can be achieved toimprove and stabilize non-tackiness properties.

(Uses of Low-friction Fluororubber Crosslinked Product

The low-friction fluororubber crosslinked product (formed product)obtained as described above is well balanced and excellent in propertiessuch as low frictional properties, low tackiness and low resilienceproperties. Therefore, it is favorably employed as:

impact-absorbing stoppers such as HDD storage head stoppers, HDDstoppers, storage head stoppers in automobile (optical) disk drives andbuilt-in camera video recorder disk drives, and printer head stoppers;

fluid (in a broad sense including gases) leak-proof rubber parts such asO-rings, packings, V-packings, oil seals, gaskets, square rings,D-rings, diaphragms and valves; and

various rubber parts such as rubber vibration insulators, belts,rubber-coated fabrics and wipers.

In particular, the low-friction fluororubber crosslinked product (formedproduct) used as the shock-absorbing stoppers represented by HDDstoppers can be expected to remarkably reduce a malfunction caused bythe tackiness of the shock-absorbing stopper to a disk arm, and to showsatisfactory damping properties at high temperatures and favorablyabsorb the vibration of the arm.

In particular, the use of the crosslinked product obtained by thepresent process as HDD magnet holder type stopper can achieve a changeof holding torque of not more than 14%, preferably not more than 10%.The change of holding torque in the above range enables stableprevention of malfunction of the HDD arm over a long term, and thestopper well satisfies other properties required.

As used herein, the change of holding torque is an indicator ofover-time change of non-tackiness of the stopper to the arm. The smallerthe value is, the longer the non-tackiness can be maintained. Namely,the stopper can prevent a malfunction of the HDD arm stably over a longterm.

INDUSTRIAL APPLICABILITY

In the invention, the fluororubber crosslinked product made from thecrosslinkable fluororubber composition of specific ingredientscomposition is heat treated. The invention thus provides thefluororubber crosslinked product having low tackiness, low frictionalproperties and low impact resilience, without deteriorating excellentproperties inherent to the fluororubbers, such as resistance to heat,oil and chemicals.

In the invention, the crosslinkable fluororubber composition of specificingredients composition is crosslinked according to need, and theresultant fluororubber crosslinked product is subjected to the specific“heat treatment”. Alternatively, the uncrosslinked fluororubbercomposition of specific ingredients composition is directly subjected tothe specific “heat treatment”. Accordingly, the low-tackiness andlow-friction fluororubber crosslinked product can be obtained stably andinexpensively in comparison with the traditional processes involvingcoating, chemical treatment and electron-beam treatment for the surfaceof fluororubber crosslinked formed product.

In particular, the fluororubber composition containing PTFE can give alow-friction fluororubber crosslinked product further improved in lowtackiness, low frictional properties and low impact resilience.

In particular, the low-friction fluororubber crosslinked product may beproduced such that the surface thereof is unleveled, for example in adepth of 0.5 to 200 μm, to reduce the contact area between thelow-friction fluororubber crosslinked product and the mating part.Consequently, the friction coefficient can be reduced to less than 1,and the low-friction fluororubber crosslinked product can display stablelow tackiness and low frictional properties.

The fluororubber crosslinked products obtained by the above-describedprocess possess the properties aforementioned and are favorably employedas rubber vibration insulators, belts, rubber-coated fabrics, wipers,fluid leak-proof rubber parts represented by O-rings and packings, andimpact-absorbing stoppers for printer head controllers and HDD (harddisk drive) head controllers, more particularly stoppers fitted in HDDto prevent a malfunction of a reading arm.

Further, the use of the rubber composition of the invention enablesefficient and inexpensive production of superior non-stickiness stopperswithout adversely affecting the environment.

EXAMPLES

The production process of the low-friction fluororubber crosslinkedproduct according to the present invention will be described in moredetail with reference to the following examples, but it should beconstrued that the invention is in no way limited to the examples.

Example 1

(Composition of Ingredients of Polyol-crosslinkable FluororubberComposition)

Fluororubber (Viton A-500 manufactured by DuPont Dow Elastomers, Mooneyviscosity ML₁₊₁₀ (121° C.): 45) . . . 100 parts by weight

FEF carbon black (SEAST GSO manufactured by TOKAI CARBON CO., LTD.,average particle diameter: 43 mμ, specific surface area: 42 m²/g) . . .2 parts by weight

Magnesium oxide (Kyowa Mag 150 manufactured by Kyowa Chemical IndustryCo., Ltd.) . . . 3 parts by weight

Calcium hydroxide (CALDIC 2000 manufactured by Ohmi Chemical Industry,Ltd., specific surface area: 17 m²/g) . . . 3 parts by weight

Bisphenol AF (manufactured by Riedel Dehaen) . . . 2.3 parts by weight

Triphenylbenzylphosphonium chloride (reagent manufactured by KantoKagaku) . . . 2.3 parts by weight

Carnauba wax (VPA No. 2 manufactured by DuPont Dow Elastomers, meltingpoint: 80° C.) . . . 1 part by weight

The above ingredients were kneaded using a kneader and an open roll millat 80° C. for 20 minutes, and the kneaded product was compression moldedusing a compression molding machine at 180° C. for 30 minutes. Theproduct formed (compression molded product) was then heat treated in anoven at 230° C. for 24 hours.

The heat-treated product (fluororubber crosslinked and heat-treatedproduct) was tested for properties such as static friction coefficient,dynamic friction coefficient, hardness, impact resilience, tackiness (1)and (2), and non-tackiness durability.

The results are shown in Table 1.

Example 2

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that the composition of ingredientsof polyol-crosslinkable fluororubber composition was changed as follows(namely, the composition was the same as in Example 1 except that theamount of triphenylbenzylphosphonium chloride was altered to 9.2 partsby weight).

The results are shown in Table 1.

(Composition of Ingredients of Polyol-crosslinkable FluororubberComposition)

Fluororubber (Viton A-500 manufactured by DuPont Dow Elastomers, Mooneyviscosity ML₁₊₁₀ (121° C.): 45) . . . 100 parts by weight

FEF carbon black (SEAST GSO manufactured by TOKAI CARBON CO., LTD.,average particle diameter: 43 mμ, specific surface area: 42 m²/g) . . .2 parts by weight

Magnesium oxide (Kyowa Mag 150 manufactured by Kyowa Chemical IndustryCo., Ltd.) . . . 3 parts by weight

Calcium hydroxide (CALDIC 2000 manufactured by Ohmi Chemical Industry,Ltd., specific surface area: 17 m²/g) . . . 3 parts by weight

Bisphenol AF (manufactured by Riedel Dehaen) . . . 2.3 parts by weight

Triphenylbenzylphosphonium chloride (reagent manufactured by KantoKagaku) . . . 9.2 parts by weight

Carnauba wax (VPA No. 2 manufactured by DuPont Dow Elastomers, meltingpoint: 80° C.) 1 part by weight

Example 3

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that PTFE was added in an amount of40 parts by weight (phr).

The results are shown in Table 1.

Example 4

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that a mold having a surfaceunleveled with an average depth of 20 μm was used in the crosslinkingmolding.

The results are shown in Table 1.

Example 5

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that the heat treatment wasperformed after glass beads (diameter: 100 μm) were shot to a surface ofthe compression molded product to roughen the surface.

The results are shown in Table 1.

Example 6

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that the compression molded productwas heat treated at 260° C. for 10 hours.

The results are shown in Table 1.

Example 7

(Composition of Ingredients of Polyol-crosslinkable FluororubberComposition)

Fluororubber (Viton A-500 manufactured by DuPont Dow Elastomers, Mooneyviscosity ML₁₊₁₀ (121° C.): 45) . . . 100 parts by weight

FEF carbon black (SEAST GSO manufactured by TOKAI CARBON CO., LTD.,average particle diameter: 43 mμ, specific surface area: 42 m²/g) . . .2 parts by weight

Magnesium oxide (Kyowa Mag 150 manufactured by Kyowa Chemical IndustryCo., Ltd.) . . . 6 parts by weight

Bisphenol AF (manufactured by Riedel Dehaen) . . . 10 parts by weight

Triphenylbenzylphosphonium chloride (reagent manufactured by KantoKagaku) . . . 10 parts by weight

Carnauba wax (VPA No. 2 manufactured by DuPont Dow Elastomers, meltingpoint: 80° C.) . . . 1 part by weight

The above ingredients were kneaded using a kneader and an open roll millat 80° C. for 20 minutes, and the kneaded product was compression moldedusing a compression molding machine at 180° C. for 30 minutes. Theproduct formed (compression molded product) was then heat treated in anoven at 230° C. for 24 hours. The heat-treated product was tested forproperties in the same manner as in Example 1.

The results are shown in Table 1.

Example 8

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that the amount of calcium hydroxidewas altered to 8 parts by weight (phr).

The results are shown in Table 1.

Example 9

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that the amount oftriphenylbenzylphosphonium chloride was altered to 6.9 parts by weight(phr).

The results are shown in Table 1.

Example 10

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that the composition of ingredientsof polyol-crosslinkable fluororubber composition was changed as follows(namely, the composition was the same as in Example 1 except that theamount of triphenylbenzylphosphonium chloride was altered to 2.76 partsby weight).

The results are shown in Table 1.

(Composition of Ingredients of Polyol-crosslinkable FluororubberComposition)

Fluororubber (Viton A-500 manufactured by DuPont Dow Elastomers, Mooneyviscosity ML₁₊₁₀ (121° C.): 45) . . . 100 parts by weight

FEF carbon black (SEAST GSO manufactured by TOKAI CARBON CO., LTD.,average particle diameter: 43 mμ, specific surface area: 42 m²/g) . . .2 parts by weight

Magnesium oxide (Kyowa Mag 150 manufactured by Kyowa Chemical IndustryCo., Ltd.) . . . 3 parts by weight

Calcium hydroxide (CALDIC 2000 manufactured by Ohmi Chemical Industry,Ltd., specific surface area: 17 m²/g) . . . 3 parts by weight

Bisphenol AF (manufactured by Riedel Dehaen) . . . 2.3 parts by weight

Triphenylbenzylphosphonium chloride (reagent manufactured by KantoKagaku) . . . 2.76 parts by weight

Carnauba wax (VPA No. 2 manufactured by DuPont Dow Elastomers, meltingpoint: 80° C.) . . . 1 part by weight

Comparative Example 1

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that the amounts oftriphenylbenzylphosphonium chloride and calcium hydroxide were alteredto 1.3 parts by weight (phr) and 6 parts by weight (phr), respectively.

The results are shown in Table 1.

Comparative Example 2

A crosslinked formed product was obtained and tested for properties inthe same manner as in Example 1, except that the crosslinked formedproduct (compression molded product) was not heat treated.

The results are shown in Table 1.

Comparative Example 3

A heat-treated product was obtained and tested for properties in thesame manner as in Comparative Example 1, except that before the heattreatment, the product formed (compression molded product) was soaked ina treatment solution containing the following crosslinking agent andcrosslinking accelerator (solvent: acetone) at a soaking temperature of20° C. and for a soaking time of 1 hour, and the product was recoveredfrom the solution, followed by removing the solvent by drying, and washeat treated at 230° C. for 24 hours.

The results are shown in Table 1.

(Treatment Solution)

Bisphenol AF (manufactured by Riedel Dehaen) . . . 10 parts by weight

Triphenylbenzylphosphonium chloride (reagent manufactured by KantoKagaku) . . . 2 parts by weight

Acetone (reagent manufactured by Kanto Kagaku) . . . 88 parts by weight

Comparative Example 4

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that the calcium hydroxide was usedin an amount of 2 parts by weight (phr), the magnesium oxide in anamount of 8 parts by weight (phr), the bisphenol AF in an amount of 1.5parts by weight (phr), and the triphenylbenzylphosphonium chloride in anamount of 0.5 part by weight (phr).

The results are shown in Table 1.

Comparative Example 5

(Composition of Ingredients of Polyol-crosslinkable FluororubberComposition)

Viton GLT (manufactured by DuPont Dow Elastomers, Mooney viscosityML₁₊₁₀ (121° C.): 90) . . . 100 parts by weight

MT carbon (Huber N-990 manufactured by Huber, average particle diameter:500 mμ, specific surface area: 6 m²/g) . . . 10 parts by weight

Calcium hydroxide (CALDIC 2000 manufactured by Ohmi Chemical Industry,Ltd., specific surface area: 17 m²/g) . . . 4 parts by weight

Triallyl isocyanurate (TAIC manufactured by Nihon Kasei CO., LTD.) . . .2.4 parts by weight

Organic peroxide crosslinking agent (PERHEXA 25B manufactured by NOFCORPORATION) . . . 0.8 part by weight

A fluororubber composition containing the above ingredients was kneadedusing a kneader and an open roll mill at 80° C. for 20 minutes, and thekneaded product was compression molded using a compression moldingmachine at 170° C. for 30 minutes. The product formed was then heattreated in an oven at 230° C. for 24 hours.

The heat-treated product was tested for properties in the same manner asin Example 1.

The results are shown in Table 1.

Comparative Example 6

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that calcium hydroxide was alteredto one having a specific surface area of 48 m²/g.

The calcium hydroxide was prepared by a method similar to that disclosedin Example 1 of paragraph [0016] of JP-A-H09-100119.

The results are shown in Table 1.

Comparative Example 7

A heat-treated product was obtained and tested for properties in thesame manner as in Example 1, except that the amounts oftriphenylbenzylphosphonium chloride and calcium hydroxide were alteredto 13.8 parts by weight (phr) and 6 parts by weight (phr), respectively.

The results are shown in Table 1.

(Conditions of Property Measurements)

The conditions of property measurements in Examples 1 to 10 andComparative Examples 1 to 7 are as follows:

(1) Measurement of Static Friction Coefficient

A rubber sheet specimen was placed on a sloping plate, and the angle ofslope was gradually increased in accordance with JIS P8147. The tangent(tan θ) of angle at which the rubber sheet started sliding wasdetermined as the static friction coefficient. The higher the staticfriction coefficient is, the higher the rubber tackiness tends to be.

(2) Measurement of Dynamic Friction Coefficient

A rubber sheet specimen 2 mm thick was tested in accordance with JIS K7125 and P 8147 with use of a surface property tester (manufactured byHEIDON) to determine the dynamic friction coefficient of sheet surface.The higher the dynamic friction coefficient is, the higher the rubbertackiness tends to be.

(Testing Conditions)

Mating part: a 10 mm diameter chrome-plated steel ball friction block

Moving rate: 50 mm/min

Load: 50 g

(3) Measurement of Hardness

A rubber sheet specimen was tested for hardness with a Type A durometerin accordance with JIS K 6253.

(4) Measurement of Impact Resilience

A laminate consisting of 6 rubber sheet specimens each 2 mm in thicknessand 29 mm in diameter was tested in accordance with JIS K 6255 by theLubke process at temperatures of 0° C., 25° C. and 70° C. to determinethe impact resilience at each temperature. The lower the impactresilience, the higher the absorption of arm vibration.

(5) Measurement of Tackiness

[Measurement of Tackiness (1)]

A metal rod (stainless steel made, weight: 16 g, configuration of curvedsurface contacting with the rubber: cylindrical column 3 mm in diameterand 1.5 mm in width) was pressed against a 2 mm thick rubber sheetspecimen at 60° C. over a period of 24 hours, and then at 0° C. for 24hours. The tackiness of the rubber to the metal rod was measured at 0°C.

[Measurement of Tackiness (2)]

A metal rod (stainless steel made, weight: 16 g, configuration of curvedsurface contacting with the rubber: cylindrical column 3 mm in diameterand 1.5 mm in width) was pressed against a 2 mm thick rubber sheetspecimen at 60° C. over a period of 72 hours, and then at 0° C. for 24hours. The tackiness of the rubber to the metal rod was measured at 0°C.

(6) Evaluation of Non-tackiness Durability

A hard disk drive as illustrated in FIG. 1 was prepared, which includeda 3 inch-diameter disk having an aluminum substrate and a sputtered thinfilm, a nanoslider of AlTiC thin film head arm, and a voice coil motor(VCM) fitted with two opposite polarity magnets.

A cylindrical outer stopper rubber having been water washed andultrasonic cleaned (outer diameter: 5 mm, length in axial direction: 10mm) was inserted around a stainless steel pin of this HDD such that thecenter in axial direction of the rubber was approximately aligned withthe center in longitudinal direction of the stainless steel pin(diameter: 1.5 mm, length, 20 mm, material: SUS 304), as shown in FIG.2.

Next, the arm was hit against the stopper rubber 50,000 times at roomtemperature, and the stopper rubber and the arm were held in contactwith each other at 60° C. over a period of 24 hours, and then at 0° C.for 24 hours. Thereafter, the tackiness was measured at 0° C.

(7) Flexing Test

The flexing resistance was tested in accordance with JIS K 5600-5-1(cylindrical mandrel test). A flex testing machine Type 1 (mandreldiameter: 2 mm) was used to fold 180° a 2 mm-thick rubber sheet at roomtemperature. The flexing resistance was evaluated as A when no crackswere found by visual and 25× light microscope observation, B when crackswere found by 25× light microscope observation (but no functionalproblems), and C when cracks were found by visual and 25× lightmicroscope observation.

TABLE 1 Example 1 2 3 4 5 6 7 8 9 10 Composition (parts by weight)Fluororubber (Viton A-500)^((a)) 100 100 100 100 100 100 100 100 100 100Fluororubber (Viton GLT)^((b)) — — — — — — — — — — FEF carbonblack^((c)) 2 2 2 2 2 2 2 2 2 2 MT carbon^((d)) — — — — — — — — — —Magnesium oxide 3 3 3 3 3 3 6 3 3 3 Calcium hydroxide (17 m²/g)/(48m²/g)  3/—  3/—  3/—  3/—  3/—  3/— —/—  8/—  3/—  3/— Bisphenol AF(crosslinking agent)^((e)) 2.3 2.3 2.3 2.3 2.3 2.3 10 2.3 2.3 2.3Perhexa 25B (crosslinking agent)^((f)) — — — — — — — — — —Triphenylbenzylphosphonium chloride 2.3 9.2 2.3 2.3 2.3 2.3 10 2.3 6.92.76 (crosslinking accelerator) Triallyl isocyanurate (crosslinking — —— — — — — — — — accelerator)^((g)) Carnauba wax^((h)) 1 1 1 1 1 1 1 1 11 PTFE — — 40 — — — — — — — crosslinking accelerator/crosslinking 1.04.0 1.0 1.0 1.0 1.0 1.0 1.0 3.0 1.2 agent (weight ratio) Kneading(kneader and open roll mill Perfd. Perfd. Perfd. Perfd. Perfd. Perfd.Perfd. Perfd. Perfd. Perfd. 80° C./20 min) Compression molding (°C./min) 180/30  180/30  180/30  180/30  180/30  180/30  180/30  180/30 180/30  180/30  Soak in treatment solution (20° C./1 hr)^((i)) — — — — —— — — — — Oven heat treatment (° C./hr) 230/24  230/24  230/24  230/24 230/24  260/10  230/24  230/24  230/24  230/24  Glass beadspretreatment/20 μm —/— —/— —/— Mold Beads —/— —/— —/— —/— —/— unlevelmold used pretrtm Properties of crosslinked product Static frictioncoefficient (JIS P 8147) 0.4 0.3 0.3 0.2 0.2 0.3 0.2 0.7 0.2 0.3 Dynamicfriction coefficient (JIS K7125 0.7 0.4 0.6 0.5 0.4 0.6 0.6 0.9 0.3 0.5P8147) Hardness (JIS K6253) 62 65 75 62 62 65 80 65 64 63 Impactresilience  0° C. 5 5 5 5 5 5 5 5 5 5 (%) 25° C. 10 10 8 10 10 10 11 1110 10 (JIS K 6255) 70° C. 39 35 30 39 39 41 35 43 37 37 Tackiness (1)(g) 6 6 4 3 0 6 6 15 6 6 Tackiness (2) (g) 45 10 30 35 25 20 35 70 15 20Tackiness after durability test (g) 8 6 5 4 2 6 6 20 6 7 Flexingresistance (JIS K 5600) A B A A A A B A A A Comparative Example 1 2 3 45 6 7 Composition (parts by weight) Fluororubber (Viton A-500)^((a)) 100100 100 100 — 100 100 Fluororubber (Viton GLT)^((b)) — — — — 100 — — FEFcarbon black^((c)) 2 2 2 2 — 2 2 MT carbon^((d)) — — — — 10 10 —Magnesium oxide 3 3 3 8 — — 3 Calcium hydroxide (17 m²/g)/(48 m²/g)  6/— 3/—  6/—  2/—  4/— —/3   6/— Bisphenol AF (crosslinking agent)^((e))2.3 2.3 2.3 1.5 — 2.3 2.3 Perhexa 25B (crosslinking agent)^((f)) — — — —0.8 — — Triphenylbenzylphosphonium chloride 1.3 2.3 1.3 0.5 — 2.3 13.8(crosslinking accelerator) Triallyl isocyanurate (crosslinking — — — —2.4 — — accelerator)^((g)) Carnauba wax^((h)) 1 1 1 1 — 1 1 PTFE — — — —— — — crosslinking accelerator/crosslinking 0.6 1.0 0.6 0.3 3.0 1.0 6.0agent (weight ratio) Kneading (kneader and open roll mill Perfd. Perfd.Perfd. Perfd. Perfd. Perfd. Perfd. 80° C./20 min) Compression molding (°C./min) 180/30  180/30  180/30  180/30  170/30  180/30  180/30  Soak intreatment solution (20° C./1 hr)^((i)) — — Perfd. — — — — Oven heattreatment (° C./hr) 230/24  — 230/24  230/24  230/24  230/24  230/24 Glass beads pretreatment/20 μm —/— —/— —/— —/— —/— —/— —/— unlevel moldProperties of crosslinked product Static friction coefficient (JIS P8147) 2.7 2.7 0.5 2.0 1.7 1.2 0.3 Dynamic friction coefficient (JISK7125 2.6 2 0.8 2.2 2.5 1.2 0.3 P8147) Hardness (JIS K6253) 60 57 67 6363 65 75 Impact resilience  0° C. 5 10 5 5 6 5 8 (%) 25° C. 12 15 10 1543 10 10 (JIS K 6255) 70° C. 63 60 55 67 72 55 32 Tackiness (1) (g) 590580 21 500 570 150 4 Tackiness (2) (g) 790 780 40 700 770 350 8Tackiness after durability test (g) 500 490 26 550 480 180 6 Flexingresistance (JIS K 5600) A A A A A A C <Note> ^((a))manufactured byDuPont Dow Elastomers, polyol vulcanizable, ^((b))manufactured by DuPontDow Elastomers, peroxide crosslinkable, ^((c))manufactured by TokaiCarbon, reinforcing agent, average particle diameter: 43 mμ, specificsurface area: 42 m²/g, ^((d))manufactured by Huber, reinforcing agentHuber N-990, average particle diameter: 500 mμ, specific surface area: 6m²/g, ^((e))manufactured by Riedel Dehaen, polyol crosslinking agent,^((f))manufactured by NOF Corporation, organic peroxide crosslinkingagent, ^((g))manufactured by Nippon Kasei Chemial Co., Ltd., trade name:TAIC, ^((h))manufactured by DuPont Dow Elastomers, VPA No. 2, mp: 80°C., ^((i))treatment solution (bisphenol AF 10 parts by weight +triphenylbenzylphosphonium chloride 2 parts by weight + acetone 88 partsby weight)

Example 11

(Composition of Ingredients of Polyol-crosslinkable FluororubberComposition)

Fluororubber (Viton A-500 manufactured by DuPont Dow Elastomers, Mooneyviscosity ML₁₊₁₀ (121° C.): 45) . . . 100 parts by weight

MT carbon (Huber N-990 manufactured by Huber, average particle diameter:500 mμ, specific surface area: 6 m²/g) . . . 20 parts by weight

Magnesium oxide (Kyowa Mag 150 manufactured by Kyowa Chemical IndustryCo., Ltd.) . . . 3 parts by weight

Calcium hydroxide (CALDIC 2000 manufactured by Ohmi Chemical Industry,Ltd., specific surface area: 17 m²/g) . . . 3 parts by weight

CURATIVE VC 30 (manufactured by DuPont Dow Elastomers, containing 50 wt% of crosslinking agent (bisphenol AF) and 50 wt % of fluororubber(Viton E-45) . . . 4.5 parts by weight

CURATIVE VC 20 (manufactured by DuPont Dow Elastomers, containing 33 wt% of crosslinking accelerator (organic phosphonium salt) and 67 wt % offluororubber (Viton E-45)) . . . 7.0 parts by weight

The above ingredients were kneaded using a kneader and an open roll millat 80° C. for 20 minutes, and the kneaded product was compression moldedusing a compression molding machine at 170° C. for 20 minutes into arubber sheet and a product (magnet holder-type stopper). The productsformed (compression molded products) were then heat treated in an ovenat 240° C. for 10 hours. The heat-treated products (fluororubbercrosslinked and heat-treated products) were tested for properties suchas hardness, change of tackiness in a magnet test and change of holdingtorque.

The results are shown in Table 2. The combination of dihydroxy aromaticcompound and organic phosphonium salt in specific amounts is shown toprovide a non-tackiness effect. Examples resulted in smaller changes ofholding torque as compared with those of conventional rubber crosslinkedproducts.

Example 12

A heat-treated product was obtained and tested for properties in thesame manner as in Example 11, except that the composition of ingredientsof polyol-crosslinkable fluororubber composition was changed as follows(namely, the composition was the same as in Example 11 except that theamount of CURATIVE VC 20 was altered to 7.5 parts by weight).

The results are shown in Table 2.

(Composition of Ingredients of Polyol-crosslinkable FluororubberComposition)

Fluororubber (Viton A-500 manufactured by DuPont Dow Elastomers, Mooneyviscosity ML₁₊₁₀ (121° C.): 45) . . . 100 parts by weight

MT carbon (Huber N-990 manufactured by Huber, average particle diameter:500 mμ, specific surface area: 6 m²/g) . . . 20 parts by weight

Magnesium oxide (Kyowa Mag 150 manufactured by Kyowa Chemical IndustryCo., Ltd.) . . . 3 parts by weight

Calcium hydroxide (CALDIC 2000 manufactured by Ohmi Chemical Industry,Ltd., specific surface area: 17 m²/g) . . . 3 parts by weight

CURATIVE VC 30 (manufactured by DuPont Dow Elastomers, containing 50 wt% of crosslinking agent (bisphenol AF) and 50 wt % of fluororubber(Viton E-45) . . . 4.5 parts by weight

CURATIVE VC 20 (manufactured by DuPont Dow Elastomers, containing 33 wt% of crosslinking accelerator (organic phosphonium salt) and 67 wt % offluororubber (Viton E-45)) . . . 8.0 parts by weight

Example 13

A heat-treated product was obtained and tested for properties in thesame manner as in Example 11, except that the amount of CURATIVE VC 20was altered to 9.0 parts by weight.

The results are shown in Table 2.

Example 14

A heat-treated product was obtained and tested for properties in thesame manner as in Example 11, except that the amount of CURATIVE VC 20was altered to 6.0 parts by weight.

The results are shown in Table 2.

Example 15

A heat-treated product was obtained and tested for properties in thesame manner as in Example 11, except that the amount of CURATIVE VC 20was altered to 10.5 parts by weight.

The results are shown in Table 2.

Comparative Example 8

A heat-treated product was obtained and tested for properties in thesame manner as in Example 11, except that the amount of CURATIVE VC 20was altered to 1.5 parts by weight.

The results are shown in Table 2. The composition of ingredients in thisexample is similar to those of general-purpose fluororubbers. Thisexample resulted in great change of holding torque.

Comparative Example 9

Fluororubber (Viton A-500 manufactured by DuPont Dow Elastomers, Mooneyviscosity ML₁₊₁₀ (121° C.): 45) . . . 100 parts by weight

MT carbon (Huber N-990 manufactured by Huber, average particle diameter:500 mμ, specific surface area: 6 m²/g) . . . 20 parts by weight

Magnesium oxide (Kyowa Mag 150 manufactured by Kyowa Chemical IndustryCo., Ltd.) . . . 3 parts by weight

Calcium hydroxide (CALDIC 2000 manufactured by Ohmi Chemical Industry,Ltd., specific surface area: 17 m²/g) . . . 3 parts by weight

Wet silica (NIPSEAL ER manufactured by NIPPON SILICA GLASS Co.,Ltd.,average particle diameter: 11 μm) . . . 1 part by weight

CURATIVE VC 30 (manufactured by DuPont Dow Elastomers, containing 50 wt% of crosslinking agent (bisphenol AF) and 50 wt % of fluororubber(Viton E-45)) . . . 4.5 parts by weight

CURATIVE VC 20 (manufactured by DuPont Dow Elastomers, containing 33 wt% of crosslinking accelerator (organic phosphonium salt) and 67 wt % offluororubber (Viton E-45)) . . . 1.5 parts by weight

A heat-treated product was obtained and tested for properties in thesame manner as in Example 11.

The results are shown in Table 2. The use of silica is shown to resultin great change of holding torque.

Comparative Example 10

A heat-treated product was obtained and tested for properties in thesame manner as in Example 11, except that the amount of CURATIVE VC 20was altered to 3.0 parts by weight.

The results are shown in Table 2. The composition of ingredients in thisexample is similar to those of general-purpose fluororubbers. Thisexample resulted in great change of holding torque.

Comparative Example 11

A heat-treated product was obtained and tested for properties in thesame manner as in Example 11, except that the amount of CURATIVE VC 20was altered to 4.5 parts by weight.

The results are shown in Table 2. The composition of ingredients in thisexample is similar to those of general-purpose fluororubbers. Thisexample resulted in great change of holding torque.

Reference Example 1

Fluororubber (Viton A-500 manufactured by DuPont Dow Elastomers, Mooneyviscosity ML₁₊₁₀ (121° C.): 45) . . . 100 parts by weight

MT carbon (Huber N-990 manufactured by Huber, average particle diameter:500 mg, specific surface area: 6 m²/g) . . . 20 parts by weight

Magnesium oxide (Kyowa Mag 150 manufactured by Kyowa Chemical IndustryCo., Ltd.) . . . 3 parts by weight

Calcium hydroxide (CALDIC 2000 manufactured by Ohmi Chemical Industry,Ltd., specific surface area: 17 m²/g) . . . 3 parts by weight

Wet silica (NIPSEAL ER manufactured by NIPPON SILICA GLASS Co.,Ltd.,average particle diameter: 11 μm) . . . 1 part by weight

CURATIVE VC 30 (manufactured by DuPont Dow Elastomers, containing 50 wt% of crosslinking agent (bisphenol AF) and 50 wt % of fluororubber(Viton E-45)) . . . 1.5 parts by weight

CURATIVE VC 20 (manufactured by DuPont Dow Elastomers, containing 33 wt% of crosslinking accelerator (organic phosphoniumsalt) and 67 wt % offluororubber (VitonE-45)) . . . 4 parts by weight

A heat-treated product was obtained and tested for properties in thesame manner as in Example 11.

The results are shown in Table 2. The use of silica is shown to resultin great change of holding torque.

(Conditions of Property Measurements)

The conditions of property measurements in Examples 11 to 15 andComparative Examples 8 to 11 are as follows:

(1) Measurement of Hardness

A rubber sheet specimen was tested for hardness with a Type A durometerin accordance with JIS K 6253.

(2) Test of Magnet Tackiness Change

A rubber sheet specimen 0.4 mm thick by 3 mm long by 3 mm wide wasplaced on a magnet whose bottom was fixed as shown in FIG. 3 (permanentmagnet, rectangular column 3.6 mm thick by 3 mm long by 3 mm wide). Ametal rod (SPCC (cold rolled steel plate), weight: 30 g, configurationof part contacting with the rubber: 3 mm×1 mm rectangular column) wasput on the rubber sheet specimen. The initial tackiness F₁ between therubber and the metal rod was measured at 23° C. and 50% humidity.

Subsequently, the metal rod was put again on the rubber specimen, andthe test unit consisting of the magnet, rubber specimen and metal rodwas allowed to stand at 60° C. and 80% humidity for 10 hours.

The unit was then brought back to 23° C. and 50% humidity, and thetackiness F₁′ after application of the moisture load was measured. Thetackiness increase was obtained from the results F₁ and F₁′ using thefollowing equation:(Tackiness increase)=(F ₁ ′−F ₁)/F ₁×100(3) Change of Holding Torque

A crosslinked product was prepared in the shape of magnet holder-typestopper as illustrated in FIG. 4, and a magnet was fitted therein. Theresultant unit was installed in a hard disk drive, and a HDD arm wasbrought into contact with the stopper. The arm was then separated fromthe stopper at 23° C. and 50% humidity, and the load required forseparation was determined as the initial holding torque F₂.

Subsequently, the arm was brought into contact with the stopper again,and they were held in contact with each other at 60° C. and 80% humidityfor 10 hours.

The stopper and the arm were then brought back to 23° C. and 50%humidity, and the holding torque F₂′ after application of the moistureload was measured. The change of holding torque was obtained from theresults F₂ and F₂′ using the following equation:(Change of holding torque)=(F ₂ ′−F ₂)/F ₂×100

TABLE 2 Reference Example Comparative Example Example 11 12 13 14 15 8 910 11 1 Composition of ingredients of fluororubber composition (parts byweight) Fluororubber (Viton A-500)⁽¹⁾ 100 100 100 100 100 100 100 100100 100 MT carbon⁽²⁾ 20 20 20 20 20 20 20 20 20 20 Magnesium oxide 3 3 33 3 3 3 3 3 3 Calcium hydroxide (17 m²/g) 3 3 3 3 3 3 3 3 3 3 NIPSEAL ER— — — — — — 1 — — 1 CURATIVE VC 30⁽³⁾ 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.54.5 1.5 (crosslinking agent- containing master batch) CURATIVE VC 20⁽⁴⁾7.0 8.0 9.0 6.0 10.5 1.5 1.5 3.0 4.5 4.0 (crosslinking accelerator-containing master batch) Crosslinking accelerator/ 1.0 1.2 1.3 0.9 1.50.2 0.2 0.4 0.7 1.8 crosslinking agent (weight ratio) Kneading (kneader80° C./20 min) Perfd. Perfd. Perfd. Perfd. Perfd. Perfd. Perfd. Perfd.Perfd. Perfd. Compression molding (° C./min) 170/20 170/20 Oven heattreatment (° C./hr) 240/10 240/10 Properties of crosslinked productHardness (JIS K 6253) 78 78 78 78 78 77 78 77 77 75 Magnetic tackinesschange (%) 13 8 5 5 9 53 70 55 45 60 Holding torque change (%) 13 7 5 68 60 80 58 48 70 ⁽¹⁾Manufactured by DuPont Dow Elastomers, polyolcrosslinkable, ⁽²⁾Manufactured by Huber, Huber N-990, average particlediameter: 500 mμ, specific surface area 6 m²/g, reinforcing agent,⁽³⁾Manufactured by DuPont Dow Elastomers, containing 50 wt % ofcrosslinking agent (bisphenol AF) and 50 wt % of fluororubber (VitonE-45) ⁽⁴⁾Manufactured by DuPont Dow Elastomers, containing 33 wt % ofcrosslinking accelerator (organic phosphonium salt) and 67 wt % offluororubber (Viton E-45)

1. A process for producing a low-friction fluororubber crosslinkedproduct, comprising: preliminarily polyol crosslinking apolyol-crosslinkable fluororubber composition according to need, thecomposition comprising a polyol-crosslinkable fluororubber incombination with a crosslinking accelerator, a polyol crosslinkingagent, calcium hydroxide and optionally magnesium oxide, wherein thecrosslinking accelerator has a weight ratio (R) to the polyolcrosslinking agent (crosslinking accelerator/polyol crosslinking agent)in the range of 0.9 to 5; and heat treating the fluororubber compositionat a temperature in the range of 150 to 300° C. for 0.1 to 48 hours toproduce a low-friction fluororubber crosslinked product having a surfacefrication coefficient of less than
 1. 2. The process according to claim1, wherein the weight ratio R is in the range of 0.9 to 3 and the heattreatment is performed at a temperature in the range of 200 to 300° C.3. The process according to claim 1, wherein the weight ratio R is inthe range of 0.9 to 2 and the heat treatment is performed at atemperature in the range of 240 to 300° C. for 10 to 48 hours.
 4. Theprocess according to claim 1, wherein the crosslinking accelerator is anorganic quaternary phosphonium salt and the polyol crosslinking agent isa bisphenol.
 5. The process according to claim 1, wherein thefluororubber composition contains the crosslinking accelerator and thepolyol crosslinking agent in amounts of 2.1 to 20 parts by weight and0.4 to 20 parts by weight, respectively, per 100 parts by weight of thepolyol-crosslinkable fluororubber.
 6. The process according to claim 1,wherein the fluororubber composition contains calcium hydroxide having aspecific surface area of less than 20 m^(2/)g in an amount of 0.5 to 10parts by weight per 100 parts by weight of the polyol-crosslinkablefluororubber.
 7. The process according to claim 1, wherein thefluororubber composition contains the magnesium oxide in an amount ofnot more than 3.0 parts by weight per 100 parts by weight of thepolyol-crosslinkable fluororubber.
 8. The process according to claim 1,wherein the polyol-crosslinkable fluororubber composition containspolytetrafluoroethylene (PTFE) in an amount of 5 to 200 parts by weightper 100 parts by weight of the polyol-crosslinkable fluororubber.
 9. Theprocess according to claim 1, wherein the polyol-crosslinkablefluororubber composition is polyol-crosslinked using a compression moldwhose inner peripheral surface is unleveled to give a crosslinkedproduct having an uneven surface with an average depth of 0.5 to 200 mm,and the crosslinked product is subjected to the heat treatment.
 10. Afluororubber composition capable of giving a crosslinked product by heattreatment that is used as a stopper in hard disk drive, the compositioncomprising a polyol-crosslinkable fluororubber, a polyol crosslinkingagent and a crosslinking accelerator, wherein the polyol crosslinkingagent is contained in an amount of 0.4 to 20 parts by weight per 100parts by weight of the polyol-crosslinkable fluororubber, and whereinthe weight ratio R of the crosslinking accelerator to the polyolcrosslinking agent (crosslinking accelerator/polyol crosslinking agent)is in the range of 0.9 to 2.0.
 11. An impact-absorbing stopper obtainedby the process for producing a low-friction fluororubber crosslinkedproduct according to claim
 1. 12. A stopper in hard disk drive obtainedby the process for producing a low-friction fluororubber crosslinkedproduct according to claim
 1. 13. The stopper in hard disk driveaccording to claim 12, wherein the stopper when used as a magnetholder-type stopper has a change of holding torque of not more than 14%.